1. Field of the Invention
The present invention relates generally to power conversion and particularly to inverter modules for use in power conversion systems.
2. Technical Background
A power conversion system typically refers to a system that converts energy obtained from a naturally occurring energy source into electricity. Examples of naturally occurring energy sources include, inter alia, oil, coal, natural gas, nuclear, hydro, wind and solar. A photovoltaic (PV) system refers to a system that employs solar panels that convert light energy into electrical energy. PV systems come in many different sizes. Small PV systems may be used to provide electrical power to small isolated devices such as lights. On the other hand, the PV system may be coupled to the electrical power grid to thereby supply the energy needs of many users.
The solar panel PV modules generate direct current (DC) power. The PV modules in a given array are typically connected in series to obtain a specified voltage. These series arrangements are often referred to in the art as “strings.” Subsequently, the various PV strings are then connected in parallel in order to obtain the specified current. If the PV system is tied to the grid, the system includes an inverter system that is configured to convert the DC power into alternating current (AC) power. The term “grid” refers to a public electricity grid and therefore, a grid tied PV system provides the public electricity grid with AC power.
An inverter is typically comprised of cascaded electronic switching devices such as insulated gate bipolar transistors (IGBTs). Each electronic switch in the switching network generates a pulse when it is actuated by its control system. The various pulses in the network are combined to form a stepped staircase waveform that approximates a sinusoidal waveform. One drawback to this approach is the following rule of thumb: the closer the stepped output voltage approaches a pure sinusoid, the more expensive the inverter becomes.
Using photovoltaic systems to provide power to the utility grid is becoming more attractive in light of the world-wide increase in the demand for power. Referring to FIGS. 1(a)-1(d), four charts provide a historical overview of conventional PV inverters. The grid tied inverter is one of the key components of a PV power conversion system, and there are essentially three types: the centralized inverter system shown in FIG. 1(a); FIG. 1 (b) illustrates string technology and FIG. 1 (c) illustrates multi-string technology; and FIG. 1 (d) illustrates AC-module and AC cell technologies. Each of these approaches has advantages and disadvantages that must be carefully considered. As such, implementations of these systems often represent a compromise of various system attributes such as harmonic rejection capability, simplicity, efficiency, flexibility, reliability, safety, modularity, and cost.
For medium power applications, the most suitable configuration is considered to be the string or multi-string technologies shown in FIGS. 1(b)-(c), where one or more strings of PV cells are connected to a single inverter. Unlike the centralized configuration, this topology offers the flexibility to optimize the number of strings and inverters for the specific application power level to increase the overall efficiency and to reduce losses. A multi-string system is a combination of several PV strings with a grid-connected inverter and is seen by many as a promising solution to the aforementioned compromises because it promises to simultaneously achieve benefits such as flexible design, ease of enlargement and high efficiency.
Referring to FIG. 2, a diagrammatic depiction of a related art grid-connected inverter system 2 is shown. The inverter 2 shown in FIG. 2 is a transformerless half-bridge diode-clamped three-level inverter. Essentially, the control system turns switches S1 and S2 ON to provide a positive output voltage, whereas switches S2 and S3 are turned ON to provide a zero voltage output. Finally, when switches S3 and S4 are turned ON, a negative voltage is provided. The main drawback to this topology is that the first string (#1) is only loaded during positive grid voltage, whereas the second string (#2) is only loaded for negative grid voltage. Accordingly, the decoupling capacitors (C1 and C2) must be relatively large. Moreover, this topology lacks modularity because it is difficult to add additional strings to boost the voltage level because each string is loaded differently.
FIG. 3 is directed to another related art photovoltaic inverter 3. Inverter 3 includes a two level voltage source inverter (VSI) that interfaces two PV strings. In comparison with the system of FIG. 2, the switching frequency of the inverter 3 must be doubled in order to use a grid inductor of the same or similar size because it can only produce a two-level output voltage. The advantages are that an individual maximum peak power tracker (MPPT) can be applied to each string and further enlargement is easily achieved by adding another PV string plus a transistor, a capacitor, and an inductor. The drawback of this topology is its buck characteristic; that is the minimum input voltage always must be larger than the maximum grid voltage.
In reference to FIG. 4, a diagrammatic depiction of a conventional three-string inverter 4 is shown. In this related art inverter, the dc-dc converters are based on current-source full-bridge inverters with an embedded HF transformer and bridge rectifier. The PV strings are easily connected to the system ground and should allow the system to be enlarged. However, in practice, it is difficult to increase the power rate since the configuration of the grid interface inverter is fixed and effectively constrains system expansion.
What is needed is an inverter system that provides a modular solution with improved power efficiency. An inverter system that adopts the “plug and play” concept is also needed. In other words, system designers would welcome and appreciate a system that easily accommodates additional PV strings and inverter modules to increase the power rate. In fact, such a system provides the designer with the freedom to tailor the system to any user specification, whether its power requirements are large or small. A system that optimizes both design flexibility and simplicity by providing.